1. Field of the Invention
The present invention generally relates to contrast enhancement, and more particularly to adaptable and adjustable transfer curve for contrast enhancement.
2. Description of the Prior Art
Contrast enhancement is one of the important digital image processing operations. The contrast enhancement operation improves the visual quality of an image with poor intensity distribution. For example, a severely faded paper is very low in contrast whose intensity distribution is concentrated in limited grey values.
Traditional histogram equalization is widely used to redistribute or equalize the histogram of the entire image. However, this method is computation intensive and therefore a slow operation. Binary contrast enhancement, or thresholding, is also traditionally used. In this operation, all pixels of brightness less than the threshold are set to black (0), and those above the threshold are set to white (255). As the binary contrast enhancement creates a very high-contrast image, it has limited application of, for example, recognizing the contour or shape of object(s) in the image. Accordingly, a contrast enhancement generally has a transfer curve rather than the sharp change in the binary contrast enhancement.
Lookup table (LUT) is one approach to the transfer curve in contrast enhancement. However, the LUT method disadvantageously requires a great amount of memories, and retrieving data from the memories results in more access cycles. Piecewise linear approximation is another approach to the transfer curve in contrast enhancement. Nevertheless, the piecewise linear method needs a number of registers for storing end points, and likely incurs approximation errors.
Accordingly, a need has arisen to propose a fast and simple way for contrast enhancement.